FIELD
[0001] The present disclosure relates to a secondary battery, a battery pack, a vehicle,
and a stationary power supply.
BACKGROUND
[0002] Nonaqueous electrolyte batteries, in which a carbon material or a lithium-titanium
oxide is used as a negative electrode active material and a layered oxide including
nickel, cobalt, manganese, and the like is used as a positive electrode active material,
secondary batteries in particular, have already been put to practical use as a power
source in a wide range of fields. Modes of such nonaqueous electrolyte batteries span
over far ranges from small-sized batteries for various electronic devices to large-sized
batteries for electric automobiles and the like. In an electrolyte for such secondary
batteries, unlike a nickel-hydrogen battery or a lead storage battery, used is a nonaqueous
organic solvent, in which ethylene carbonate, methyl ethyl carbonate, and the like
are mixed. The electrolyte using such a solvent has oxidation resistance and reduction
resistance that are higher than those of an aqueous electrolyte, and thus electrolysis
of the solvent hardly occurs. For that reason, the nonaqueous secondary battery can
realize a high electromotive force of from 2 V to 4.5 V.
[0003] On the other hand, many of the organic solvents are flammable materials, and thus
the safety of the nonaqueous secondary battery is apt to be inferior to the secondary
battery using an aqueous solution, in principle. Although various measures are being
taken to improve the safety of the secondary battery using the electrolyte of organic
solvent base, such measures are not necessarily sufficient. Furthermore, for the nonaqueous
secondary battery, a dry environment is necessary in the production process, and thus
the production cost is consequently increased. In addition, the electrolyte of organic
solvent base has inferior electro-conductivity, and thus the internal resistance of
the nonaqueous secondary battery is apt to increase. These have been big issues in
applications for an electric automobile and a hybrid electric automobile in which
the battery safety and the battery cost are emphasized, and in an application for
a large-sized storage battery for electricity storage. In order to solve the problems
of the nonaqueous secondary battery, a secondary battery using an aqueous electrolyte
has been proposed. However, the active material may easily become dislodged from the
current collector due to the electrolysis of the aqueous electrolyte, and hence the
operation of the secondary battery is not stable, which has been problematic in performing
satisfactory charge and discharge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004]
FIG. 1 is an SEM image of an example of a negative electrode surface in a first approach;
FIG. 2 is a partially cutout cross-sectional view schematically showing an example
of a secondary battery according to the first approach;
FIG. 3 is a side view of the battery of FIG. 2;
FIG. 4 is a partially cutout perspective view schematically showing another example
of the secondary battery according to the first approach;
FIG. 5 is an enlarged cross-sectional view of section A in FIG. 4;
FIG. 6 is a perspective view schematically showing an example of a battery module
according to a second approach;
FIG. 7 is a perspective view schematically showing an example of a battery pack according
to a third approach;
FIG. 8 is an exploded perspective view schematically showing another example of the
battery pack according to the third approach;
FIG. 9 is a block diagram showing an electric circuit of the battery pack shown in
FIG. 8;
FIG. 10 is a cross-sectional view schematically showing an example of a vehicle according
to a fourth approach;
FIG. 11 is a diagram schematically showing another example of the vehicle according
to the fourth approach; and
FIG. 12 is a block diagram showing an example of a system including a stationary power
supply according to a fifth approach.
DETAILED DESCRIPTION
[0005] According to one approach, provided is a secondary battery including an aqueous electrolyte,
a positive electrode, and a negative electrode. A compound containing an element A
is present on at least a part of the surface of the negative electrode. The element
A is at least one selected from the group consisting of Hg, Pb, Zn, and Bi. According
to scanning electron microscopy, a region where the compound containing the element
A is present accounts for 50% or more of the surface of the negative electrode.
[0006] According to another approach, a battery pack including the secondary battery according
to the above approach is provided.
[0007] According to still another approach, a vehicle including the battery pack according
to the above approach is provided.
[0008] According to still another approach, a stationary power supply including the battery
pack according to the above approach is provided.
[0009] According to the above approaches, provided is a secondary battery using an aqueous
electrolyte with high charge-discharge efficiency and storage performance, a battery
pack with high charge-discharge efficiency and storage performance, and a vehicle
and a stationary power supply including the battery pack.
[0010] Hereinafter, approaches will be described with reference to the drawings. Note that
the same reference numerals are given to common configurations throughout the approaches,
and redundant descriptions are omitted. In addition, each drawing is a schematic view
for describing the approach and facilitating the understanding thereof, and there
are some differences in shape, dimension, ratio, and the like from an actual device.
These, however, may be changed as appropriate, considering the following description
and known technology.
(First Approach)
[0011] In order to solve the issues of nonaqueous secondary batteries, conversion of the
electrolyte to an aqueous solution has been considered. In an aqueous solution electrolyte,
a potential range of performing charging and discharging of the battery is required
to be limited within a potential range where an electrolysis reaction of water, included
as a solvent, does not occur. For example, when a lithium manganese oxide is used
as a positive electrode active material and a lithium vanadium oxide is used as a
negative electrode active material, electrolysis of the aqueous solvent can be avoided.
According to this combination, however, though an electromotive force of about 1 V
to 1.5 V can be obtained, it is difficult to obtain an energy density sufficient for
a battery.
[0012] When a lithium manganese oxide is used as the positive electrode active material
and a lithium titanium oxide such as LiTi
2O
4 or Li
4Ti
5O
12 is used as the negative electrode active material, an electromotive force of about
2.6 V to 2.7 V can be theoretically obtained, and thus the battery can be expected
to be attractive in terms of the energy density. In a nonaqueous lithium ion battery
employing such a combination of the positive and negative electrode materials, excellent
life performance can be obtained, and such a battery has already been put into practical
use. In an aqueous solution electrolyte, however, since a potential of lithium insertion
and extraction for the lithium titanium oxide is about 1.5 V (vs. Li/Li
+) relative to a lithium reference potential, electrolysis of the aqueous solution
electrolyte is apt to occur. In particular, at the negative electrode, hydrogen is
vigorously generated by electrolysis occurring on a surface of a negative electrode
current collector or a metal outer can electrically connected to the negative electrode,
and thus the active material may become easily flaked off from the current collector
due to the influence of the hydrogen generation. As a result, such a battery does
not function stably and sufficient charge and discharge cannot be performed.
[0013] In the prior art, it had been possible to provide a lithium secondary battery containing
zinc in a current collector so as to have a sufficient energy density, be excellent
in charge-discharge efficiency and lifetime performance, and be inexpensive and highly
safe, but there has been room for improvement in terms of charge-discharge efficiency
and storage performance.
[0014] As a result of earnest studies to solve this problem, the inventors arrived at a
secondary battery according to the first approach.
[0015] A secondary battery according to a first approach includes an aqueous electrolyte,
a positive electrode, and a negative electrode. A compound containing an element A
is present on at least a part of the surface of the negative electrode. The element
A is at least one selected from the group consisting of Hg, Pb, Zn, and Bi. A region,
in which the compound containing the element A is observed, accounts for 50% or more
of regions on the surface of the negative electrode which are observed by a scanning
electron microscope.
[0016] This surface is a portion observed when the negative electrode is observed by a method
described later. The compound containing the element A is present on the surface of
the negative electrode of the secondary battery according to the present approach.
Specifically, the phrase "being present on the surface of the negative electrode"
means that the compound containing the element A is present on the surfaces of at
least one of a negative electrode active material, an electro-conductive agent for
the negative electrode, and a binder. The compound containing the element A indicates
a formation of at least one of a sole metal, an oxide, a chloride, a nitrate, a sulfate,
and hydroxide of the element A, which has precipitated on the above surface(s) as
such a compound containing the element A. For the sake of convenience, the sole metal
is also referred to as a compound here. The compound containing the element A is present
physically. The compound containing the element A has a small exchange current density
and a high hydrogen generation overvoltage. Therefore, the hydrogen generation can
be suppressed in the negative electrode.
[0017] It is considered that the compound containing the element A present on the surface
of the negative electrode is present as a uniform layer on a part of the surface of
the negative electrode as shown in FIG. 1. In FIG. 1, a portion appearing white is
considered to be the compound containing the element A, and a portion appearing black
is considered to be an exposed negative electrode portion. In the secondary battery
according to the approach, the compound containing the element A can be made present
as a uniform thin layer in the negative electrode. Therefore, the compound containing
the element A hardly becomes a cause for electrical resistance even when covering
a wide range of the surface of the negative electrode. In the portion where the compound
containing the element A is present among the negative electrode surface, the negative
electrode active material is not directly in contact with the electrolyte, but is
in contact with the electrolyte via the compound containing the element A with a high
hydrogen overvoltage. Thereby, it is possible to suppress the hydrogen generation
in the negative electrode and improve the storage performance.
[0018] Further, by the presence of the compound containing the element A on the surface
as a uniform thin layer, it is possible to improve the adhesion between the negative
electrode active materials, and thereby to suppress flaking off from the electrode.
Further, with the layer being uniform, it is possible to suppress variations in electrical
resistance within the electrode, and thereby to prevent state of charge (SOC) deviation
in the battery and local deterioration in the electrode due to local heat generation.
The compound containing the element A is present as a uniform layer on at least a
part of the surface of the negative electrode, so that the proportion of the compound
containing the element A present on the surface of the negative electrode of the secondary
battery according to the present approach is 50% or greater and 100% or less.
[0019] Since the proportion of the compound containing the element A present on the surface
of the negative electrode is from 50% to 100% as thus described, it is possible to
suppress the hydrogen generation from the negative electrode and improve the charging-discharging
efficiency and the storage performance.
[0020] Further, it is more preferable that the proportion of the compound containing the
element A present on the surface of the negative electrode be 70% or more. This is
because, within this range, the hydrogen generation from the negative electrode can
be suppressed more efficiently, and the adhesion between the negative electrode active
materials can be improved, thereby suppressing flaking-off of the electrode active
material. This enables the rate performance to be maintained and the storage performance
to be further improved.
[0021] When the proportion of the compound containing the element A present on the surface
of the negative electrode is less than 50%, the compound containing the element A
does not sufficiently cover the negative electrode surface. As a consequence, there
is not obtained the effect of improvement in the binding property between the current
collector and the negative electrode by the compound containing the element A. As
another consequence, hydrogen generation is not suppressed so that dislodging of the
negative electrode active material off the current collector is caused by generated
hydrogen bubbles, leading to deterioration in the cycle performance. In addition,
there arise variations in the electrical resistance in the electrode, caused by bias
of portions in which the compound containing the element A is present on the surface
of the negative electrode. This causes progress of SOC deviation in the electrode
and progress of local deterioration in the electrode due to local heat generation,
and hence a proportion less than 50% is not preferred.
[0022] As thus described, when the compound containing the element A is present on the surfaces
of the negative electrode active material, the electro-conductive agent, and the binder,
the adhesion to each other can be improved. Further, since the compound containing
the element A can thinly cover the surface of the negative electrode, it is possible
to prevent direct contact between the negative electrode active material and the electrolyte.
Moreover, since the negative electrode active material is in contact with the electrolyte
via the compound containing the element A which has a high hydrogen overvoltage, it
is possible to suppress the hydrogen generation in the negative electrode and improve
the storage performance. Therefore, in the secondary battery according to the present
approach, it is possible to suppress the hydrogen generation from the negative electrode,
improve the binding property of each of the negative electrode active material, the
electro-conductive agent, the binder, and the current collector, and increase the
electronic conduction paths, thereby enabling improvement in the cycle performance
and reduction in the electrical resistance. Therefore, the charge-discharge efficiency
and the storage performance can be improved.
[0023] Although the description has been given of the presence of the compound containing
the element A on the surface of the negative electrode, the compound containing the
element A may also be made present on the surfaces of negative electrode active material,
the electro-conductive agent, and the binder that are inside the negative electrode.
Because the compound containing the element A may also be made present inside, it
is possible to suppress the hydrogen generation from the inside of the electrode.
It is thus possible to prevent reduction in number of electro-conductive paths caused
by detachment of the active material or cracking of the electrodes due to the hydrogen
generation, and is therefore possible to further improve the cycle performance.
[0024] Further, Regarding the state of the presence of the compound containing the element
A on the surface of the negative electrode and the abundance thereof on the surface,
important are, for example, a charging rate at the time of the initial charge, the
composition of the electrolyte, and the number of charge-discharge cycles. In addition,
an amount of metal contained in the electrolyte is also important, and the concentration
of the element A in the electrolyte is also an important parameter in performing metal
coating. When the element A is not added into the electrolyte and, for example, the
compound containing the element A is used as the current collector, the amount of
the element A in the electrolyte can be controlled by regulating the time from the
assembly of the battery to the initial charge and discharge.
[0025] Here, a flow in which the compound containing element A is precipitated on the surface
will be described.
[0026] A secondary battery containing the element A in the negative electrode current collector
is assembled. At this time, the element A is eluted into the electrolyte. By controlling
the time until performing the initial charge and discharge of the assembled secondary
battery, the element A eluted in the electrolyte precipitates on the negative electrode
surface, such as surfaces of the negative electrode active material and the like during
the charge reaction. The precipitated element A forms a compound containing the element
A by the charge reaction, and the compound precipitates on the surface. In the secondary
battery according to the present approach, the standby time is set short and the initial
charge-discharge rate is set low, thereby enabling precipitation of the compound containing
the element A on the surface of the negative electrode. Also in the case where the
element A is put into the electrolyte, it is possible to form the compound containing
the element A by the charge reaction in a similar manner, and precipitate the formed
compound on the surface of the negative electrode. As a simple method, for example,
addition of a surfactant into the electrolyte can be mentioned. Addition of the surfactant
enables the surface shape control and the film property control. In the surface shape
control, the precipitation reaction of the compound containing the element A can be
suppressed due to the suppressing action by deposition of the surfactant onto the
plating surface or by electric consumption. The hardness and electric conductivity
of the plating film can be changed by the film property control. The surfactant will
be described later.
[0027] In the secondary battery according to the present approach, by containing the element
A, a layer in which the compound containing the element A is uniform can be made present
on the surface of the negative electrode. Therefore, it is possible to suppress displacement
of the negative electrode active material off the negative electrode current collector.
Further, a uniform layer can be made present on a part of the negative electrode surface,
and thus, it is possible to suppress the electrical resistance caused by the compound
containing the element A being present on the surface. As a result, even at a potential
in the vicinity of 1.5 V (vs. Li/Li
+) with a lithium potential as a reference, it is possible to charge and discharge
oxides of titanium, lithium titanium oxides, and lithium titanium composite oxides
in the aqueous solvent. Note that, although discussion has been made regarding lithium,
sodium may be used in place of lithium. The element A can be contained in, for example,
the current collector, the electrolyte, and the negative electrode in the secondary
battery.
[0028] Here, a method for measuring the proportion of the compound containing element A
present on the negative electrode will be described.
[0029] First, the secondary battery is disassembled. For example, after the secondary battery,
having undergone the initial charge, is discharged, this battery is disassembled and
the negative electrode is taken out. The negative electrode having been taken out
is washed with pure water for 30 minutes, and then vacuum-dried for 24 hours in an
environment at a temperature of 80°C. After drying, the temperature is returned to
25°C and the negative electrode is taken out.
[0030] The negative electrode taken out in this way is analyzed using a scanning electron
microscope (SEM) image described below, and structural features of the negative electrode
are clarified. Equipment and analysis conditions used for the SEM observation and
software used for the image analysis are as follows:
[SEM equipment and analysis conditions]
[0031] Instruct Name = SU8000 (manufactured by Hitachi High-Technologies Corporation)
Serial Number = HI-0946-0005
Data Number = SU8020
Signal Name = LAI00(U)
SE Det Setting = LA-BSE, U, Even, VSE = 100
Accelerating Voltage = 3000 Volt
Deceleration Voltage = 0 Volt
Magnification = 50000
Working Distance = 2600 um
LensMode = High
Condition: Vacc = 3 kV, Mag = x 50.0k, WD = 2.6 mm
[SEM image analysis software]
Photo Impact (manufactured by Corel Corporation)
Image-Pro plus (manufactured by Media Cybernetics, Inc.)
[0032] The image analysis method will be described below. First, an SEM image of the surface
of the negative electrode is obtained by SEM observation. The contrast and the brightness
of the SEM image is adjusted using the image editing software Photo Impact, such that
a portion in which the negative electrode is observed, namely, a portion in which
the compound containing the element A is not present, would be displayed black, and
a portion in which the compound containing the element A is observed would be displayed
white. The image obtained in this way was subjected to an automatic measurement function
(measurement of the ratio between the black portion and the white portion) by using
the image analysis software Image-Pro Plus. Thereby, obtained is the proportion of
a region A in the total of the region A and a region B (region A ÷ (region A + region
B) × 100), the region A being the region where the compound containing the element
A is observed, and the region B being the region where the negative electrode is observed
(where the compound containing the element A is not present).
[0033] One third or more of the area of the negative electrode is measured at a magnification
of 30 times or more, and the proportion of the region is obtained for each image.
An average value of the remaining 60% excluding the upper 20% and the lower 20% is
taken as the proportion of the region A in the total of the region A and the region
B on the surface of the negative electrode.
[0034] Although the presence of the compound containing the element A on the surface of
the negative electrode has been described so far, the compound containing the element
A can be present also on the negative electrode current collector. Details will be
described later.
[0035] Materials for the respective members usable in the secondary battery according to
the first approach will be described in detail.
1) Negative Electrode
[0036] The negative electrode includes a negative electrode current collector, and a negative
electrode active material layer disposed on the negative electrode current collector.
The negative electrode active material layer is disposed on at least one surface of
the negative electrode current collector. For example, the negative electrode active
material layer may be disposed on one surface of the negative electrode current collector,
or negative electrode active material layers may be disposed on one surface of the
negative electrode current collector and on a surface on the reverse side.
[0037] The negative electrode active material layer includes a negative electrode active
material including at least one compound selected from the group consisting of an
oxide of titanium, a lithium titanium oxide, and a lithium titanium composite oxide.
The oxides may be used alone or as a mixture of several oxides. In the oxides, Li
insertion and extraction reaction occurs within a range of 1 V to 2 V (vs. Li/Li
+) relative to a lithium reference potential. For that reason, when the oxides described
above are used as the negative electrode active material of the lithium secondary
battery, the change in volume due to expansion and contraction, which accompany charge
and discharge, is small, and thus long operation life can be realized.
[0038] It is preferable that at least one element selected from the group consisting of
Zn, Ga, In, Bi, Tl, Sn, Pb, Ti and Al, is included in the current collector. These
elements may be used alone or as a mixture of multiple elements, and may be included
in the state of a metal or metal alloy. The metal and metal alloy may be included
alone or as a mixture of two or more. When such an element is included in the current
collector, the mechanical strength of the current collector is increased and thus
the processing thereof is improved. Further, the effect of suppressing the electrolysis
of the aqueous solvent to thereby suppress the hydrogen generation is increased. Of
the elements described above, Zn, Pb, Ti and Al are more preferable.
[0039] The current collector is, for example, a metal foil made of these metals. Further,
the current collector is, for example, a foil made of an alloy containing these metals.
Such a foil may contain, for example, one or more elements described below, in addition
to the element A. Besides the foil, examples of the shape of the metal article or
alloy article as the current collector include a mesh and a porous structure. For
improving the energy density and output, the shape of the foil having a small volume
and a large surface area is desired.
[0040] Further, the negative electrode current collector may include a substrate containing
a metal different from the element A. In such a case, the hydrogen generation can
be suppressed by the presence of the compound containing the element A on at least
a part of the surface of the substrate. The compound containing the element A present
on the surface is desirably disposed so as to be in contact with the negative electrode
active material layer. For example, the element A can be made present on the surface
of the substrate by plating the compound containing element A on the substrate. Alternatively,
it is possible to perform plating treatment using an alloy containing element A on
the surface of the substrate.
[0041] The current collector may include at least one compound selected from the group consisting
of an oxide of element A, a hydroxide of element A, a basic carbonate compound of
element A, and a sulfate compound of element A. The oxide of element A and/or the
hydroxide of element A and/or the basic carbonate compound of element A, and/or the
sulfate compound of element A are preferably included in at least a part of the surface
region of the current collector, within a depth region of from 5 nm to 1 µm in the
depth direction from the current collector surface. An example of the oxide of element
A includes ZnO, an example of the hydroxide of element A includes Zn(OH)
2, an example of the basic carbonate compound of element A includes 2ZnCO
3·3Zn(OH)
2, and an example of the sulfate compound of element A includes ZnSO
4·7H
2O, and the like.
[0042] When at least one of an oxide of the element A, a hydroxide of the element A, a basic
carbonate compound of the element A, and a sulfate compound of the element A is present
in the surface layer portion of the current collector, hydrogen generation can be
suppressed. In addition, when these compounds are present in the surface layer portion
of the current collector, the adhesion among the current collector, the active material,
the electro-conductive agent, and the binder is improved, enabling an increase in
paths for electronic conduction. Therefore, it is possible to improve the cycle performance
and reduce the electrical resistance.
[0043] The substrate preferably includes at least one metal selected from the group consisting
of Al, Fe, Cu, Ni, and Ti. The metals may be included in the state of an alloy. The
substrate may include the metal or metal alloy alone or as a mixture of two or more.
The substrate preferably includes Al, Ti, or an alloy thereof, from the perspective
of weight reduction.
[0044] Whether or not at least one compound selected from the group consisting of the oxide
of element A, the hydroxide of element A, the basic carbonate compound of element
A, and the sulfate compound of element A is contained in the current collector, can
be examined by disassembling the battery as described above and then conducting inductively
coupled plasma (ICP) emission spectrometry.
[0045] The negative electrode active material includes one compound, or two or more compounds
selected from the group consisting of an oxide of titanium, lithium titanium oxide,
and lithium titanium composite oxide. Examples of the lithium titanium composite oxide
include a niobium titanium oxide and a sodium niobium titanium oxide. The compounds
desirably have a Li insertion potential in a range of 1 V (vs. Li/Li
+) to 3 V (vs. Li/Li
+).
[0046] Examples of the oxide of titanium include an oxide of titanium having a monoclinic
structure, an oxide of titanium having a rutile structure, and an oxide of titanium
having an anatase structure. For the oxide of titanium having each crystal structure,
the composition before charging can be represented by TiO
2, and the composition after charging can be represented by Li
xTiO
2, wherein x is 0 ≤ x ≤ 1. The structure before charging for the oxide of titanium
having the monoclinic structure can be represented by TiO
2(B).
[0047] Examples of the lithium titanium oxide include a lithium titanium oxide having a
spinel structure (for example, the general formula: Li
4+xTi
5O
12 wherein x is -1 ≤ x ≤ 3), a lithium titanium oxide having a ramsdellite structure
(for example, Li
2+xTi
3O
7 wherein -1 ≤ x ≤ 3), Li
1+xTi
2O
4 wherein 0 ≤ x ≤ 1, Li
1.1+xTi
1.8O
4 wherein 0 ≤ x ≤ 1, Li
1.07+xTi
1.86O
4 wherein 0 ≤ x ≤ 1, and Li
xTiO
2 wherein 0 < x ≤ 1), and the like. The lithium titanium oxide includes, for example,
a lithium titanium composite oxide in which a dopant is introduced into the above
lithium titanium oxide having the spinel structure or the ramsdellite structure.
[0048] Examples of the niobium titanium oxide include oxides represented by Li
aTiM
bNb
2±βO
7±σ wherein 0 ≤ a ≤ 5, 0 ≤ b ≤ 0.3, 0 ≤ β ≤ 0.3, 0 ≤ σ ≤ 0.3, and M includes at least
one selected from the group consisting of Fe, V, Mo, and Ta.
[0049] Examples of the sodium niobium titanium oxide include an orthorhombic Na-containing
niobium titanium composite oxide represented by the general formula Li
2+vNa
2-wM1
xTi
6-y-zNb
yM2
zO
14+δ wherein 0 ≤ v ≤ 4, 0 < w < 2, 0 ≤ x< 2, 0 < y < 6, 0 ≤ z < 3, y + z < 6, -0.5 ≤ δ
≤ 0.5, M1 includes at least one selected from the group consisting of Cs, K, Sr, Ba,
and Ca, and M2 includes at least one selected from the group consisting of Zr, Sn,
V, Ta, Mo, W, Fe, Co, Mn, and A1.
[0050] Preferable compounds for the negative electrode active material may include the oxide
of titanium having an anatase structure, the oxide of titanium having a monoclinic
structure, and the lithium titanium oxide having the spinel structure. Each compound
has a Li insertion potential of from 1.4 V (vs. Li/Li
+) to 2 V (vs. Li/Li
+), and thus, when combined with a lithium manganese oxide as the positive electrode
active material, for example, a high electromotive force can be obtained. Especially,
the lithium titanium oxide having the spinel structure is more preferable because
the change in volume due to the charge-discharge reaction is very small.
[0051] The negative electrode active material may be included in the negative electrode
active material layer in the form of particles. The negative electrode active material
particle may be singular primary particles, secondary particles in which each of the
secondary particles include aggregated primary particles, or a mixture of singular
primary particles and secondary particles. The shape of the particles is not particularly
limited and, for example, may be a spherical shape, an elliptic shape, a flat shape,
a fiber shape, or the like.
[0052] It is preferable that an average particle size (a diameter) of the secondary particles
of the negative electrode active material is 3 µm or more, more preferably from 5
µm to 20 µm. When the size is within this range, the surface area of the active material
is small, and thus the effect of suppressing the hydrogen generation can be increased.
[0053] The negative electrode active material having the secondary particles whose average
particle size is 3 µm or more can be obtained, for example, by the following method.
First, starting materials for the active material are subjected to synthetic reaction
to produce an active material precursor having an average particle size of 1 µm or
less. After that, the active material precursor is subjected to a firing treatment,
followed by a pulverization treatment using a pulverizer such as a ball mill or a
jet mill. Next, by a firing treatment, the active material precursors are aggregated
and grown into secondary particles having a large particle size.
[0054] The primary particles in the negative electrode active material desirably have an
average particle size of 1 µm or less. This way, a diffusion length of Li ions within
the active material is shortened, and a specific surface area becomes larger. Therefore,
high input performance (rapid charging performance) can be obtained. On the other
hand, when the average particle size is small, the particles become easily aggregated,
whereby the distribution of the electrolyte becomes inclined toward the negative electrode,
and the electrolyte may consequently be exhausted at the positive electrode. For that
reason, the lower limit of the average primary particle size is desirably 0.001 µm.
The average particle size is more preferably from 0.1 µm to 0.8 µm.
[0055] The negative electrode active material particles desirably have a specific surface
area of from 3 m
2/g to 200 m
2/g, as determined by a BET method employing N
2 deposition. By having such a specific surface area, the affinity between the negative
electrode and the electrolyte can be further enhanced.
[0056] The negative electrode active material layer (excluding the current collector) desirably
has a specific surface area of from 3 m
2/g to 50 m
2/g. The specific surface area is more preferably from 5 m
2/g to 50 m
2/g. The negative electrode active material layer may be a porous layer including the
negative electrode active material, the electro-conductive agent, and the binder,
where the layer is supported on the current collector.
[0057] The porosity of the negative electrode (excluding the current collector) is desirably
in a range of 20% to 50%, whereby a negative electrode having excellent affinity between
the negative electrode and the electrolyte and high density can be obtained. The porosity
is more preferably in a range of 25% to 40%.
[0058] The electro-conductive agent may include carbon materials such as acetylene black,
carbon black, coke, carbon fiber, and graphite, and powders of a metal such as nickel
or zinc. The electro-conductive agent may be used alone or as a mixture of two or
more agents. It is desirable to use the metal powder as the electro-conductive agent,
because hydrogen is generated from the carbon material itself.
[0059] The binder may include, for example, polytetrafluoroethylene (PTFE), polyvinylidene
fluoride (PVdF), fluororubber, ethylene-butadiene rubber, polypropylene (PP), polyethylene
(PE), carboxymethyl cellulose (CMC), polyimide (PI), polyacrylimide (PAI), and the
like. The binder may be used alone or as a mixture of two or more binders.
[0060] With respect to the mixing ratio of the negative electrode active material, the electro-conductive
agent, and the binder in the negative electrode active material layer, it is preferable
that the negative electrode active material is included in a range of 70% by weight
to 95% by weight, the electro-conductive agent is included in a range of 3% by weight
to 20% by weight, and the binder is included in a range of 2% by weight to 10% by
weight. When the mixing ratio of the electro-conductive agent is 3% by weight or more,
the electro-conductivity of the negative electrode can be made good, and when the
mixing ratio is 20% by weight or less, the degradation of the electrolyte on the surface
of the electro-conductive agent can be reduced. When the mixing ratio of the binder
is 2% by weight or more, sufficient electrode strength can be obtained, and when the
mixing ratio is 10% by weight or less, the insulating portions within the electrode
can be reduced.
[0061] The negative electrode can be produced, for example, by the following method. First,
the negative electrode active material, the electro-conductive agent, and the binder
are dispersed in an appropriate solvent to prepare a slurry. The resulting slurry
is coated onto the current collector, and the coat of applied slurry is dried to form
the negative electrode active material layer on the current collector. Here, for example,
the slurry may be coated on one side of the current collector, or may be coated on
one surface of the current collector and a surface on the reverse side. Then, the
current collector and the negative electrode active material layer are subjected to
pressing, for example, heat pressing, whereby the negative electrode can be produced.
2) Positive Electrode
[0062] The positive electrode may include a positive electrode current collector, and a
positive electrode active material layer supported on one surface or both of reverse
surfaces of the positive electrode current collector, where the positive electrode
active material layer includes an active material, an electro-conductive agent, and
a binder.
[0063] It is preferable to use a foil, porous structure, or mesh made of a metal such as
stainless steel, Al, or Ti as the positive electrode current collector. In order to
prevent corrosion of the current collector caused by the reaction of the current collector
with the electrolyte, the surface of the current collector may be covered with another
element.
[0064] As the positive electrode active material, there may be used a material capable of
having lithium and sodium be inserted and extracted. The positive electrode may include
one kind of positive electrode active material, or include two or more kinds of positive
electrode active materials. Examples of the positive electrode active material include
a lithium manganese composite oxide, a lithium nickel composite oxide, a lithium cobalt
aluminum composite oxide, a lithium nickel cobalt manganese composite oxide, a spinel
type lithium manganese nickel composite oxide, a lithium manganese cobalt composite
oxide, a lithium iron oxide, a lithium fluorinated iron sulfate, a phosphate compound
having an olivine crystal structure (for example, Li
xFePO
4 wherein 0 ≤ x ≤ 1, or Li
xMnPO
4 wherein 0 ≤ x ≤ 1), and the like. The phosphate compound having an olivine crystal
structure has excellent thermal stability.
[0065] Examples of the positive electrode active material with which a high positive electrode
potential can be obtained are described below. Examples include lithium manganese
composite oxides such as Li
xMn
2O
4 having a spinel structure wherein 0 < x ≤ 1, or Li
xMnO
2 wherein 0 < x ≤ 1; a lithium nickel aluminum composite oxide such as Li
xNi
1-yAl
yO
2 wherein 0 < x ≤ 1 and 0 < y ≤ 1; lithium cobalt composite oxides such as Li
xCoO
2 wherein 0 < x ≤ 1; lithium nickel cobalt composite oxides such as Li
xNi
1-y-zCo
yMn
zO
2 wherein 0 < x ≤ 1, 0 < y ≤ 1, and 0 ≤ z ≤ 1; lithium manganese cobalt composite oxides
such as Li
xMn
yCo
1-yO
2 wherein 0 < x ≤ 1 and 0 < y ≤ 1; spinel type lithium manganese nickel composite oxides
such as Li
xMn
2-yNi
yO
4 wherein 0 < x ≤ 1 and 0 < y < 2; lithium phosphates having an olivine structure such
as Li
xFePO
4 wherein 0 < x ≤ 1, Li
xFe
1-yMn
yPO
4 wherein 0 < x ≤ 1 and 0 ≤ y ≤ 1, or Li
xCoPO
4 wherein 0 < x ≤ 1; fluorinated iron sulfates such as Li
xeSO
4F wherein 0 < x ≤ 1, and the like.
[0066] Further examples of the positive electrode active material include sodium manganese
composite oxide, sodium nickel composite oxide, sodium cobalt composite oxide, sodium
nickel cobalt manganese composite oxide, sodium iron composite oxide, sodium phosphate
compounds (for example, sodium iron phosphate and sodium vanadium phosphate), sodium
iron manganese composite oxide, sodium nickel titanium composite oxide, sodium nickel
iron composite oxide, and sodium nickel manganese composite oxide.
[0067] Examples of a preferable positive electrode active material include iron composite
oxides (for example, Na
yFeO
2, wherein 0 ≤ y ≤ 1), iron manganese composite oxides (for example, Na
yFe
1-xMn
xO
2, wherein 0 < x < 1, 0 ≤ y ≤ 1), nickel titanium composite oxide (for example, Na
yNi
1-xTi
xO
2, wherein 0 < x < 1, 0 ≤ y ≤ 1), a nickel iron composite oxide (for example, Na
yNi
1-xFe
xCO
2, wherein 0 < x < 1, 0 ≤ y ≤ 1), nickel-manganese composite oxide (for example, Na
yNi
1-xMn
xO
2, wherein 0 < x < 1, 0 ≤ y ≤ 1), nickel manganese iron composite oxide (for example,
Na
yNi
1-x-zMn
xFe
zCO
2, wherein 0 < x < 1, 0 ≤ y ≤ 1, 0 < z < 1, 0 < 1-x-z < 1), iron phosphate (for example,
Na
yFePO
4, wherein 0 ≤ y ≤ 1).
[0068] The particle of the positive electrode active material may be singular primary particles,
secondary particles in which each of the secondary particles include aggregated primary
particles, and a mixture of both the singular primary particles and the secondary
particles. The primary particles of the positive electrode active material preferably
have an average particle size (a diameter) of 10 µm or less, more preferably from
0.1 µm to 5 µm. The secondary particles of the positive electrode active material
preferably have an average particle size (a diameter) of 100 µm or less, more preferably
from 10 µm to 50 µm.
[0069] It is preferable that at least a part of the particle surface of the positive electrode
active material is covered with a carbon material. The carbon material may be in the
form of a layered structure, a particulate structure, or a form of aggregated particles.
[0070] As the electro-conductive agent for increasing the electron conductivity of the positive
electrode layer and suppressing the contact resistance between the positive electrode
layer and the current collector, examples include acetylene black, carbon black, graphite,
carbon fiber having an average fiber diameter of 1 µm or less, and the like. The electro-conductive
agent may be used alone or as a mixture of two or more agents.
[0071] The binder for binding the active material to the electro-conductive agent include,
for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber,
ethylene-butadiene rubber, styrenebutadiene rubber (SBR), polypropylene (PP), polyethylene
(PE), carboxymethyl cellulose (CMC), polyimide (PI), and polyacrylimide (PAI). The
binder may be used alone or as a mixture of two or more binders.
[0072] With respect to the mixing ratio of the positive electrode active material, the electro-conductive
agent, and the binder in the positive electrode active material layer, it is preferable
that the positive electrode active material is included in a range of 70% by weight
to 95% by weight, the electro-conductive agent is included in a range of 3% by weight
to 20% by weight, and the binder is included in a range of 2% by weight to 10% by
weight. When the mixing ratio of the electro-conductive agent is 3% by weight or more,
the electro-conductivity of the positive electrode can be made good, and when the
mixing ratio is 20% by weight or less, the degradation of the electrolyte on the surface
of the electro-conductive agent can be reduced. When the mixing ratio of the binder
is 2% by weight or more, sufficient electrode strength can be obtained, and when the
mixing ratio is 10% by weight or less, the insulating portions within the electrode
can be reduced.
[0073] The positive electrode can be produced, for example, by the following method. First,
the positive electrode active material, the electro-conductive agent, and the binder
are dispersed in an appropriate solvent to prepare a slurry. The resulting slurry
is coated onto the current collector, and the coat of applied slurry is dried to form
the positive electrode active material layer on the current collector. Here, for example,
the slurry may be coated on one side of the current collector, or may be coated on
one surface of the current collector and a surface on the reverse side. Then, the
current collector and the positive electrode active material layer are subjected to
pressing, for example, heat pressing, whereby the positive electrode can be produced.
3) Electrolyte
[0074] Examples of the electrolyte include an electrolytic solution containing an aqueous
solvent and a first electrolyte, and a gel electrolyte obtained by combining a polymer
material in this electrolytic solution. The polymer material includes, for example,
polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO),
and the like. The electrolytic solution will be described here. The electrolyte contains
at least one anion selected from the group consisting of NO
3-, Cl-, LiSO
4-, SO
42-, and OH
-.
[0075] The electrolyte may contain one of these anions, or alternatively, two or more anions
may be included. In order to distinguish an electrolyte as used for generically naming
the electrolytic solution and the gel electrolyte, from an electrolyte as a solute,
the electrolyte as the solute is referred to as a first electrolyte for the sake of
convenience.
[0076] As an aqueous solvent, a solution including water may be used. Here, the solution
including water may be pure water, or alternatively, a mixed solution or a mixed solvent
of water and materials other than water.
[0077] In the above-described electrolyte, the amount of water solvent (for example, amount
of water in the aqueous solvent) is preferably 1 mol or more, based on 1 mol of salt
as solute. The amount of water solvent is more preferably 3.5 mol or more, based on
1 mol of salt as solute.
[0078] As the first electrolyte, there may be used a substance that becomes dissociated
and thus generates the anion described above when the substance is dissolved in water,
in a solution containing water, or in a solvent containing water. In particular, preferable
are lithium salts that dissociate into Li ion(s) and the anion described above. Such
lithium salts include, for example, LiNO
3, LiCl, Li
2SO
4, LiOH, and the like.
[0079] The lithium salt that dissociates into Li ion(s) and the above anion has a relatively
high solubility in aqueous solvents. For that reason, there can be obtained an electrolyte,
in which the anion concentration is of a high concentration of from 1 M to 10 M, and
thus having favorable Li ion diffusibility.
[0080] The electrolyte containing NO
3- and/or Cl
- may be used in a wide anion concentration range of about 0.1 M to 10 M. From the
perspective of fulfilling both ion conductivity and lithium equilibrium potential,
the anion concentration is preferably of a high concentration of from 3 M to 12 M.
It is more preferable that the anion concentration of the electrolyte containing NO
3- or Cl
- is from 8 M to 12 M.
[0081] The electrolyte containing LiSO
4- and/or SO
42- may be used in an anion concentration range of about 0.05 M to 2.5 M. From the perspective
of ion conductivity, the anion concentration is preferably of a high concentration
of from 1.5 M to 2.5 M.
[0082] The OH
- concentration in the electrolyte is desirably from 10
-10 M to 0.1 M.
[0083] The electrolyte may contain both lithium ions and sodium ions.
[0084] It is preferable that the electrolyte has a pH of from 4 to 13. When the pH is less
than 4, since the electrolyte would be acidic, degradation of the active material
is apt to progress. When the pH is more than 13, since there is decrease in an overvoltage
for oxygen generation at the positive electrode, electrolysis of the aqueous solvent
is apt to progress.
[0085] The solute in the electrolyte, i.e., the first electrolyte can be determined qualitatively
and quantitatively, for example, by ion chromatography. Ion chromatography is particularly
preferable as the analysis method because of high sensitivity.
[0086] Examples of specific measurement conditions for the qualitative and quantitative
analysis of the solute included in the electrolyte according to ion chromatography
are shown below:
System: Prominence HIC-SP
Analysis Column: Shim-pack IC-SA3
Guard Column: Shim-pack IC-SA3 (G)
Eluent: 3.6 mmol/L, aqueous sodium carbonate solution
Flow Rate: 0.8 mL/minute
Column Temperature: 45°C
Injection Amount: 50 µL
Detection: electric conductivity
[0087] Whether or not water is included in the electrolyte can be examined by gas chromatography
- mass spectrometry (GC-MS) measurement. Water content in the electrolyte can be calculated,
for example, from emission spectrometry using ICP, or the like. In addition, the mole
numbers of the solvent can be calculated from the measurement of specific weight of
the electrolyte. The same electrolyte may be used on the positive electrode side and
the negative electrode side, or different electrolytes may be used.
[0088] When different electrolytes are respectively used on the positive electrode side
and the negative electrode side, the pH of the electrolyte of the positive electrode
is preferably from 1 to 7. When the pH of the electrolyte of the positive electrode
is 8 or more, the oxygen generation reaction resulting from electrolysis of water
progresses advantageously, which is not preferable. When the pH is 1 or less, degradation
of the active material proceeds, which is not preferable. The pH of the electrolyte
of the negative electrode is preferably from 7 to 14. When the pH of the electrolyte
is 7 or less, the hydrogen generation reaction due to the electrolysis of water advantageously
advances, which is not preferable.
[0089] An additive may be added to the electrolyte. For example, a surfactant or a metal
including element A may be added. The surfactant is, for example, polyoxyalkylene
alkyl ether, polyethylene glycol, polyvinyl alcohol, nonylpheyl eicosaethylene glycol
ether, thiourea, disodium 3,3'-dithiobis (1-propanphosphate), dimercaptothiadiazole,
boric acid, oxalic acid, malonic acid, saccharin, sodium naphthalene sulfonate, gelatin,
potassium nitrate, aromatic aldehyde, and heterocyclic aldehyde. The surfactants may
be used alone or in combination of two or more. In addition, the added metal may be
present in the electrolyte either as an ion or as a solid.
[0090] In a case where a metal including element A is used as an additive (added metal)
to the electrolyte, when the concentration of the metal contained in the electrolyte
becomes excessive, ions cannot be exchanged between the negative electrode active
material and the electrolyte due to precipitation of the metal, and the battery may
not operate as a secondary battery. Therefore, care should be taken to prevent the
concentration of the metal in the electrolyte from becoming excessively high. Also,
care should be taken to prevent the pH of the electrolyte from greatly fluctuating
due to addition of the metal. As methods to add the element A, the metal A may be
added at the time of producing a slurry of the negative electrode, the metal A may
be added to the electrolyte, or both methods may be used at the same time. However,
when Hg among the element A is used, Hg is desirably mixed together with the active
material, the electro-conductive agent, and the binder when producing the negative
electrode The form of the metal to be added may be a single metal or may be one or
a combination of two or more of oxides, chlorides, sulfides, nitrates, sulfates, and
hydroxides.
[0091] Whether the surfactant is contained in the electrolyte can be examined using GC-MS
described above. For example, the electrolyte is extracted with hexane, and the organic
solvent in the electrolyte is separated. This separated organic solvent can be identified
by conducting GC-MS and nuclear magnetic resonance measurement (NMR). The added metal
can be examined by ICP.
4) Separator
[0092] A separator may be disposed between the positive electrode and the negative electrode.
When the separator is made from an insulating material, it is possible to prevent
electrical contact between the positive electrode and the negative electrode. In addition,
it is desirable to use a separator having a shape that allows the electrolyte to be
capable of migrating between the positive electrode and the negative electrode. Examples
of the separator include a non-woven, a film, a paper sheet, and the like. Examples
of the material forming the separator may include polyolefin such as polyethylene
and polypropylene, and cellulose. Preferable examples of the separator include a non-woven
including cellulose fiber and a porous film including a polyolefin fiber. The separator
preferably has a porosity of 60% or more. The fiber diameter is preferably 10 µm or
less. When the fiber diameter is 10 µm or less, the affinity of the separator with
the electrolyte is improved, thus resulting in decreased battery resistance. The more
preferable range of the fiber diameter is 3 µm or less. The cellulose-including non-woven
having a porosity of 60% or more can be well impregnated with the electrolyte, and
can exhibit a high output performance at a low temperature to a high temperature.
In addition, even during storage for a long time in a charged state, during float
charging, or when exposed to overcharge, the separator does not react with the negative
electrode, and short-circuiting between the negative electrode and the positive electrode
caused by precipitation of dendrites of the lithium metal does not occur. The more
preferable range is from 62% to 80%.
[0093] A solid electrolyte may also be used as the separator. As the separator, oxides such
as LATP (Li
1+xAl
xTi
2-x(PO
4)
3, where 0.1 ≤ x ≤ 0.4) having a NASICON framework, amorphous LATP (e.g., Li
2.9PO
3.3N
0.46), and garnet LLZ (e.g., Li
7La
3Zr
2O
12) are preferable.
[0094] The examples of solid electrolyte also include β alumina, Na
1+xZr
2Si
xP
3-xO
12(0 ≤ x ≤ 3), and NaAlSi
3O
8.
[0095] The separator preferably has a thickness of from 20 µm to 100 µm, and a density of
from 0.2 g/cm
3 to 0.9 g/cm
3. Within these ranges, the mechanical strength and the reduction of battery resistance
can be well-balanced, and a secondary battery having a high output and having suppressed
internal short-circuiting can be provided. In addition, there is little thermal contraction
of the separator under a high temperature environment, and the capability for the
battery to be stored under high temperature becomes good.
5) Container Member
[0096] A container made of metal, a container made of laminate film, a container made of
resin, such as polyethylene or polypropylene, may be used for a container member in
which the positive electrode, the negative electrode, and the electrolyte are housed.
[0097] As the container made of metal, a metal can made of nickel, iron, stainless steel,
or element A and having an angular or cylindrical shape may be used.
[0098] The container made of resin and the container made of metal desirably have a wall
thickness within a range of 1 mm or less, and more preferably 0.5 mm or less. An even
more preferable range is 0.3 mm or less. The lower limit of the wall thickness is
desirably 0.05 mm.
[0099] The laminate film includes, for example, a multilayer film in which a metal layer
is covered with resin layers, and the like. Examples of the metal layer include a
stainless steel foil, an aluminum foil, and an aluminum alloy foil. A polymer such
as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET)
may be used for the resin layer. The laminate film preferably has a thickness of 0.5
mm or less, more preferably 0.2 mm or less. The lower limit of the thickness of the
laminate film is desirably 0.01 mm.
[0100] The secondary battery according to the approach may be applied to secondary batteries
of various forms such as an angular shaped form, cylindrical shaped form, a flat-type,
a thin-type, or a coin-type. The secondary battery preferably has a bipolar structure,
whereby there is an advantage in that a cell having plural electrode units connected
in series can be produced with a single cell.
[0101] An example of a secondary battery according to the approach is explained with reference
to FIG. 2 to FIG. 5.
[0102] One example of a secondary battery using a container made of metal is shown in FIG.
2 and FIG. 3.
[0103] The electrode group 1 is housed in a rectangular-tube-shaped metal container 2. The
electrode group 1 has a structure in which the positive electrode 3, the negative
electrode 4, and the separator 5 disposed therebetween are spirally wound in a manner
such that a flat shape is obtained. An electrolyte (not shown) is held in the electrode
group 1. As shown in FIG. 3, a belt-shaped positive electrode lead 6 is electrically
connected to each of plural positions on the edge of the positive electrode 3 located
on the end surface of the electrode group 1. A belt-shaped negative electrode lead
7 is electrically connected to each of plural positions on the edge of the negative
electrode 4 located on the end surface. The plural positive electrode leads 6 are
bundled into one, and electrically connected to a positive electrode electro-conduction
tab 8. A positive electrode terminal is composed from the positive electrode leads
6 and the positive electrode electro-conduction tab 8. The negative electrode leads
7 are bundled into one, and connected to a negative electrode electro-conduction tab
9. A negative electrode terminal is composed from the negative electrode leads 7 and
the negative electrode electro-conduction tab 9. A metal sealing plate 10 is fixed
over an opening of the metal container 2 by welding or the like. The positive electrode
electro-conduction tab 8 and the negative electrode electro-conduction tab 9 are respectively
drawn out to the outside through outlets provided on the sealing plate 10. The inner
circumferential surface of each outlet of the sealing plate 10 is covered with an
insulating member 11, in order to avoid short-circuiting due to contact of the sealing
plate 10 with the positive electrode electro-conduction tab 8 and the negative electrode
electro-conduction tab 9.
[0104] One example of a secondary battery using a container member made of the laminate
film is shown in FIG. 4 and FIG. 5.
[0105] A stacked electrode group 1 is housed in a bag-form container 2. The bag-form container
2 is made of a laminate film where a metal layer is sandwiched between two resin films.
As shown in FIG. 5, the stacked electrode group 1 has a structure in which positive
electrodes 3 and negative electrodes 4 are alternately stacked with a separator 5
sandwiched therebetween. The electrode group 1 includes plural positive electrodes
3. Each of the plural positive electrodes 3 includes a current collector 3a, and positive
electrode active material-including layers 3b formed on both of reverse surfaces of
the positive electrode current collector 3a. The electrode group 1 includes plural
negative electrodes 4. Each of the plural negative electrodes 4 includes a current
collector 4a, and negative electrode active material-including layers 4b formed on
both of reverse surfaces of the current collector 4a. An end of the current collector
4a of each of the negative electrodes 4 protrudes out from the positive electrodes
3. The protruded current collector 4a is electrically connected to a belt-shaped negative
electrode terminal 12. The distal end of the belt-shaped negative electrode terminal
12 is extended out from the container 2. Although not shown in the drawings, an end
of the current collector 3a of the positive electrode 3 protrudes from the electrodes
4 at the side opposed to the protruded end of the current collector 4a. The current
collectors 3a protruding from the negative electrodes 4 are electrically connected
to a belt-shaped positive electrode terminal 13. The distal end of the belt-shaped
positive electrode terminal 13 is positioned on the opposite side from the negative
electrode terminal 12, and extends out from a side of the container 2.
[0106] In the secondary batteries shown in FIG. 2 to FIG. 5, there may be provided a safety
valve for releasing hydrogen gas that has generated within the container to the outside.
As the safety valve, either one of a return type valve, which operates when an internal
pressure becomes higher than a pre-determined value and functions as a sealing plug
when the internal pressure is reduced, and a non-return type valve, which does not
recover its function as the sealing plug once it is operated, can be used. Although
the secondary batteries shown in FIG. 2 to FIG. 5 are sealed batteries, an open type
battery is possible, in the case that a circulation system for converting hydrogen
gas into water is included.
[0107] According to the approach described above, a secondary battery is provided, which
includes an aqueous electrolyte, a positive electrode, and a negative electrode, wherein
a compound containing an element A is present on at least a part of the surface of
the negative electrode, the element A being at least one selected from the group consisting
of Hg, Pb, Zn, and Bi, and according to scanning electron microscopy, a region, in
which the compound containing the element A is present, accounts for 50% or more of
regions on the surface of the negative electrode. By making the compound which contains
the element A present at such an area ratio, it is possible to suppress the hydrogen
generation from the negative electrode, improve the binding property between the current
collector and the active material, and further, increase electronic conduction paths.
Hence, it is possible to provide a secondary battery having high charge-discharge
efficiency and storage performance.
(Second Approach)
[0108] According to a second approach, a battery module including a secondary battery as
a unit cell is provided. As the secondary battery, a secondary battery of the first
approach may be used.
[0109] Examples of the battery module include a battery module including plural unit cells
as structural units where has unit cells are electrically connected in series or in
parallel in each structural unit, a battery module including a unit structured by
plural unit cells that are electrically connected in series or a unit structured by
plural unit cells that are electrically connected in parallel, and the like.
[0110] The battery module may be housed in a housing. As the housing, a metal can formed
of aluminum alloy, iron, stainless steel, or the like, a plastic container, or the
like may be used. The container desirably has a wall thickness of 0.5 mm or more.
[0111] Examples of the aspect in which the plural secondary batteries are electrically connected
in series or in parallel include an aspect in which the plural secondary batteries
each has a container and are electrically connected in series or in parallel, and
an aspect in which plural electrode groups are housed in the same housing and are
electrically connected in series or in parallel. Specific examples of the former are
those in which positive electrode terminals and negative electrode terminals of plural
secondary batteries are connected via metal bus bars (for example, aluminum, nickel,
or copper). Specific examples of the latter include an aspect in which plural electrode
groups are housed in one housing in a state of being electrochemically insulated from
each other by partitions, and these electrode groups are electrically connected to
each other in series. When the number of batteries that are electrically connected
in series is in a range of 5 to 7, voltage compatibility with a lead storage battery
becomes good. In order to further increase the voltage compatibility with the lead
storage battery, a structure in which 5 or 6 unit cells are connected in series is
preferable.
[0112] One example of the battery module is explained with reference to FIG. 6. A battery
module 31, shown in FIG. 6, includes plural square-type secondary batteries 32
1 to 32
5 according to the first approach (for example, FIG. 2 or FIG. 3) as unit cells. A
positive electrode electro-conduction tab 8 of battery 32
1 and a negative electrode electro-conduction tab 9 of battery 32
2 positioned adjacent thereto, are electrically connected by a lead 33. Further, a
positive electrode electro-conduction tab 8 of the battery 32
2 and a negative electrode electro-conduction tab 9 of battery 32
3 positioned adjacent thereto, are electrically connected through a lead 33. In this
manner, the batteries 32
1 to 32
5 are connected in series.
[0113] According to the battery module of the second approach, by including the secondary
battery according to the first approach, there can be provided a battery module having
high charge-discharge efficiency and storage performance.
(Third Approach)
[0114] According to a third approach, a battery pack is provided. The battery pack includes
the secondary battery according to the first approach.
[0115] The battery pack according to the third approach may include one or more secondary
batteries (unit cells) according to the first approach described above. The plural
secondary batteries, which may be included in the battery pack according to the third
approach, may be electrically connected to each other in series, in parallel or in
a combination of in series and in parallel. The plural secondary batteries may be
electrically connected to compose a battery module. In the case of composing a battery
module from plural secondary batteries, the battery module according to the second
approach may be used.
[0116] The battery pack according to the third approach may further include a protective
circuit. The protective circuit has a function of controlling the charge and discharge
of the secondary battery (or secondary batteries). Alternatively, a circuit included
in equipment that uses the battery pack as a power source (for example, an electronic
device, a vehicle such as an automobile, or the like) may be used as the protective
circuit of the battery pack.
[0117] Moreover, the battery pack according to the third approach may further include an
external power distribution terminal. The external power distribution terminal is
configured to externally output current from the secondary battery and/or to input
current into a unit cell. In other words, when the battery pack is used as a power
source, the current is externally provided through the external power distribution
terminal. When the battery pack is charged, the charge current (including a regenerative
energy of a power of a vehicle such as an automobile, or the like) is provided to
the battery pack through the external power distribution terminal.
[0118] An example of the battery pack according to the third approach is explained with
reference to FIG. 7. FIG. 7 is a schematic perspective view showing one example of
the battery packs.
[0119] A battery pack 40 includes a battery module including the secondary battery shown
in FIGS. 4 and 5. The battery pack 40 includes a housing 41, and a battery module
42 housed in the housing 41. In the battery module 42, plural (for example, five)
secondary batteries 43
1 to 43
5 are electrically connected in series. The secondary batteries 43
1 to 43
5 are stacked in a thickness direction. The housing 41 has an opening 44 on each of
an upper portion and four side surfaces. The side surfaces, from which the positive
and negative electrode terminals 12 and 13 of the secondary batteries 43
1 to 43
5 protrude, are exposed through the opening 44 of the housing 41. A positive electrode
terminal 45 for output of the battery module 42 is belt-shaped, and one end thereof
is electrically connected to any or all of the positive electrode terminals 13 of
the secondary batteries 43
1 to 43
5, and the other end protrudes beyond the opening 44 of the housing 41 and thus protrudes
past the upper portion of the housing 41. On the other hand, a negative electrode
terminal 46 for output of the battery module 42 is belt-shaped, and one end thereof
is electrically connected to any or all of the negative electrode terminals 12 of
the secondary batteries 43
1 to 43
5, and the other end protrudes beyond the opening 44 of the housing 41 and thus protrudes
past the upper portion of the housing 41.
[0120] Another example of the battery pack according to the third approach is explained
in detail with reference to FIG. 8 and FIG. 9. FIG. 8 is an exploded perspective view
showing another example of the battery pack according to the third approach. FIG.
9 is a block diagram showing an electric circuit of the battery pack in FIG. 8.
[0121] Plural unit cells 51, i.e. flat-type secondary batteries, are stacked such that externally
extending negative electrode terminals 52 and positive electrode terminals 53 are
arranged in the same direction, and the resulting stack is fastened with an adhesive
tape 54 to form a battery module 55. The unit cells 51 are electrically connected
to each other in series, as shown in FIG. 9.
[0122] A printed wiring board 56 is disposed facing the side surfaces of the unit cells
51 from which the negative electrode terminals 52 and the positive electrode terminals
53 extend out. A thermistor 57, a protective circuit 58, and an external power distribution
terminal 59 are mounted on the printed wiring board 56, as shown in FIG. 9. An electrically
insulating plate (not shown) is attached to the surface of the printed wiring board
56 facing the battery module 55 to avoid unnecessary connection with wirings of the
battery module 55.
[0123] A positive electrode lead 60 is connected to a positive electrode terminal 53 located
lowermost in the battery module 55, and the distal end of the lead 60 is inserted
into a positive electrode-side connector 61 on the printed wiring board 56 and thus
electrically connected to the connector. A negative electrode lead 62 is connected
to a negative electrode terminal 52 located uppermost in the battery module 55, and
the distal end of the lead 62 is inserted into a negative electrode-side connector
63 on the printed wiring board 56 and thus electrically connected to the connector.
The connectors 61 and 63 are connected to the protective circuit 58 through wirings
64 and 65 formed on the printed wiring board 56.
[0124] The thermistor 57 detects the temperature of the unit cells 51, and the detection
signals are sent to the protective circuit 58. The protective circuit 58 can shut
off a plus wiring (positive electrode-side wiring) 66a and a minus wiring (negative
electrode-side wiring) 66b between the protective circuit 58 and the external power
distribution terminal 59 under predetermined conditions. A predetermined condition
is, for example, the case where the temperature detected by the thermistor 57 becomes
a predetermined temperature or higher. Another example of the predetermined condition
is the case when the over-charge, over-discharge or over-current of the unit cells
51 is detected. The detection of the over-charge, or the like, is performed for each
individual unit cell 51 or for the battery module 55. When each individual unit cell
51 is detected, the battery voltage may be detected, or the positive electrode potential
or negative electrode potential may be detected. In the latter case, a lithium electrode,
which is used as a reference electrode, is inserted into each individual unit cell
51. In the case of FIG. 8 and FIG. 9, a wiring 67 for voltage detection is connected
to each of the unit cells 51, and the detected signals are sent to the protective
circuit 58 through the wirings 67.
[0125] Protective sheets 68, made of rubber or resin, are arranged on three side planes
of the battery module 55 except for the side plane where the positive electrode terminals
53 and the negative electrode terminals 52 protrude out.
[0126] The battery module 55 is housed in a housing container 69 together with the protective
sheets 68 and the printed wiring board 56. That is, the protective sheets 68 are arranged
on both internal surfaces along a long side direction and one internal surface along
a short side direction of the housing container 69, and the printed wiring board 56
is disposed on the opposite internal surface along the short side direction. The battery
module 55 is located in a space surrounded by the protective sheets 68 and the printed
wiring board 56. A lid 70 is attached to the upper surface of the housing container
69.
[0127] In order to fix the battery module 55, a heat-shrinkable tape may be used instead
of the adhesive tape 54. In such a case, the battery module is fastened by placing
the protective sheets on both side surfaces of the battery module, revolving the heat-shrinkable
tape around the battery module, and thermally shrinking the heat-shrinkable tape.
[0128] In FIGS. 8 and 9, an aspect has been shown in which the unit cells 51 are connected
in series; however, in order to increase the battery capacity, the cells may be connected
in parallel. Alternatively, the connection in series and the connection in parallel
may be combined. Furthermore, assembled battery packs may be connected to each other
in series and/or in parallel.
[0129] The aspect of the battery pack may be appropriately changed depending on the application
thereof. The battery pack can be suitably used in applications in which cycle performance
is demanded to be excellent when large current is taken out. Specifically the battery
pack may be used, for example, as a power source of a digital camera, as a battery
for installing in a vehicle such as a two-wheeled or four-wheeled hybrid electric
automobile, a two-wheeled or four-wheeled electric automobile, a power-assisted bicycle,
or a railway car, or as a stationary battery. In particular, the battery pack is suitably
used onboard a vehicle.
[0130] In a vehicle onto which the battery pack according to the third approach has been
installed, the battery pack is configured, for example, to recover regenerative energy
from motive force of the vehicle.
[0131] According to the battery pack of the third approach described above, by including
the lithium secondary battery according to the first approach, there can be provided
a battery pack having high charge-discharge efficiency and storage performance.
(Fourth Approach)
[0132] According to a fourth approach, a vehicle is provided. The vehicle includes the battery
pack according to the third approach.
[0133] In the vehicle according to the fourth approach, the battery pack is configured,
for example, to recover regenerative energy from motive force of the vehicle. The
vehicle according to the fourth approach may include a mechanism for converting kinetic
energy of the vehicle into regenerative energy.
[0134] Examples of the vehicle according to the fourth approach include two-wheeled to four-wheeled
hybrid electric automobiles, two-wheeled to four-wheeled electric automobiles, power-assisted
bicycles, and railway cars.
[0135] In the vehicle according to the fourth approach, the installing position of the battery
pack is not particularly limited. For example, when installing the battery pack on
the vehicle, the battery pack may be installed in the engine compartment of the vehicle,
in rear parts of the vehicle, or under seats.
[0136] An example of the vehicle according to the fourth approach is explained below, with
reference to the drawings.
[0137] FIG. 10 is a cross-sectional view schematically showing an example of a vehicle according
to the fourth approach.
[0138] A vehicle 200, shown in FIG. 10 includes a vehicle body 201 and a battery pack 202.
The battery pack 202 may be the battery pack according to the third approach.
[0139] The vehicle 200, shown in FIG. 10, is a four-wheeled automobile. As the vehicle 200,
for example, a two-wheeled to four-wheeled hybrid electric automobile, a two-wheeled
to four-wheeled electric automobile, a power-assisted bicycle, or railway car may
be used.
[0140] The vehicle 200 may include plural battery packs 202. In that case, the battery packs
202 may be connected to each other in series or in parallel. The connection may be
a combination of the connection in series and the connection in parallel.
[0141] The battery pack 202 is installed in an engine compartment located at the front of
the vehicle body 201. The position at which the battery pack 202 is installed is not
particularly limited. The battery pack 202 may be installed in rear sections of the
vehicle body 201, or under a seat. The battery pack 202 may be used as a power source
of the vehicle 200. The battery pack 202 can also recover regenerative energy of motive
force of the vehicle 200.
[0142] Next, with reference to FIG. 11, an aspect of operation of the vehicle according
to the fourth approach is explained.
[0143] FIG. 11 is a view schematically showing another example of the vehicle according
to the fourth approach. A vehicle 300, shown in FIG. 11, is an electric automobile.
[0144] The vehicle 300, shown in FIG. 11, includes a vehicle body 301, a vehicle power source
302, a vehicle ECU (electric control unit) 380, which is a master controller of the
vehicle power source 302, an external terminal (an external power connection terminal)
370, an inverter 340, and a drive motor 345.
[0145] The vehicle 300 includes the vehicle power source 302, for example, in an engine
compartment, in the rear sections of the automobile body, or under a seat. In FIG.
11, the position of the secondary battery installed in the vehicle 300 is schematically
shown.
[0146] The vehicle power source 302 includes plural (for example, three) battery packs 312a,
312b and 312c, BMU (a battery management unit) 311, and a communication bus 310. The
three battery packs 312a, 312b and 312c are electrically connected in series. The
battery pack 312a includes a battery module 314a and a battery module monitoring unit
(VTM: voltage temperature monitoring) 313a. The battery pack 312b includes a battery
module 314b, and a battery module monitoring unit 313b. The battery pack 312c includes
a battery module 314c, and a battery module monitoring unit 313c. The battery packs
312a, 312b and 312c can each be independently removed, and may be exchanged by a different
battery pack 312.
[0147] Each of the battery modules 314a to 314c includes plural single-batteries connected
to each other in series. At least one of the plural single-batteries is the secondary
battery according to the first approach. The battery modules 314a to 314c each perform
charging and discharging through a positive electrode terminal 316 and a negative
electrode terminal 317.
[0148] In order to collect information concerning security of the vehicle power source 302,
the battery management unit 311 performs communication with the battery module monitoring
units 313a to 313c and collects information such as voltages or temperatures of the
single-batteries included in the battery modules 314a to 314c included in the vehicle
power source 302.
[0149] The communication bus 310 is connected between the battery management unit 311 and
the battery module monitoring units 313a to 313c. The communication bus 310 is configured
so that multiple nodes (i.e., the battery management unit and one or more battery
module monitoring units) share a set of communication lines. The communication bus
310 is, for example, a communication bus configured based on CAN (Control Area Network)
standard.
[0150] The battery module monitoring units 313a to 313c measure a voltage and a temperature
of each single-battery in the battery modules 314a to 314c based on commands communicated
from the battery management unit 311. It is possible, however, to measure the temperatures
only at several points per battery module, and the temperatures of all of the single-batteries
need not be measured.
[0151] The power source for vehicle 302 may also have an electromagnetic contactor (for
example, a switch unit 333 shown in FIG. 11) for switching connection between the
positive electrode terminal 316 and the negative electrode terminal 317. The switch
unit 333 includes a precharge switch (not shown), which is turned on when the battery
modules 314a to 314c are charged, and a main switch (not shown), which is turned on
when battery output is supplied to a load. The precharge switch and the main switch
include a relay circuit (not shown), which is turned on or off based on a signal provided
to a coil disposed near the switch elements.
[0152] The inverter 340 converts an inputted direct current voltage to a three-phase alternate
current (AC) high voltage for driving a motor. Three phase output terminal(s) of the
inverter 340 is (are) connected to each three-phase input terminal of the drive motor
345. The inverter 340 controls an output voltage based on control signals from the
battery management unit 311 or the vehicle ECU 380, which controls the entire operation
of the vehicle.
[0153] The drive motor 345 is rotated by electric power supplied from the inverter 340.
The rotation is transferred to an axle and driving wheels W via a differential gear
unit, for example.
[0154] The vehicle 300 also includes a regenerative brake mechanism, though not shown. The
regenerative brake mechanism rotates the drive motor 345 when the vehicle 300 is braked,
and converts kinetic energy to regenerative energy, as electric energy. The regenerative
energy, recovered in the regenerative brake mechanism, is inputted into the inverter
340 and converted to direct current. The direct current is inputted into the vehicle
power source 302.
[0155] One terminal of a connecting line L1 is connected via a current detector (not shown)
in the battery management unit 311 to the negative electrode terminal 317 of the vehicle
power source 302. The other terminal of the connecting line L1 is connected to a negative
electrode input terminal of the inverter 340.
[0156] One terminal of a connecting line L2 is connected through the switch unit 333 to
the positive electrode terminal 316 of the vehicle power source 302. The other terminal
of the connecting line L2 is connected to a positive electrode input terminal of the
inverter 340.
[0157] The external terminal 370 is connected to the battery management unit 311. The external
terminal 370 is able to connect, for example, to an external power source.
[0158] The vehicle ECU 380 cooperatively controls the battery management unit 311 together
with other units in response to inputs operated by a driver or the like, thereby performing
the management of the whole vehicle. Data concerning the security of the vehicle power
source 302, such as a remaining capacity of the vehicle power source 302, are transferred
between the battery management unit 311 and the vehicle ECU 380 via communication
lines.
[0159] The vehicle according to the fourth approach includes the battery pack according
to the third approach. The vehicle according to the fourth approach, therefore, is
excellent in charge-discharge efficiency and storage performance by virtue of including
the battery pack having high charge-discharge efficiency and storage performance.
Furthermore, since the battery pack has excellent life performance, a vehicle of high
reliability can be provided.
(FIFTH APPROACH)
[0160] According to a fifth approach, a stationary power supply is provided. The stationary
power supply includes a battery pack according to the third approach. Note that instead
of a battery pack according to the third approach, the stationary power supply may
have a battery module according to the second approach or a secondary battery according
to the first approach installed therein.
[0161] The stationary power supply according to the fifth approach includes a battery pack
according to the third approach. Therefore, the stationary power supply according
to the fifth approach can realize long life.
[0162] FIG. 12 is a block diagram showing an example of a system including a stationary
power supply according to the fifth approach. FIG. 12 is a diagram showing an application
example to stationary power supplies 112, 123 as an example of use of battery packs
40A, 40B according to the third approach. In the example shown in FIG. 12, a system
110 in which the stationary power supplies 112, 123 are used is shown. The system
110 includes an electric power plant 111, the stationary power supply 112, a customer
side electric power system 113, and an energy management system (EMS) 115. Also, an
electric power network 116 and a communication network 117 are formed in the system
110, and the electric power plant 111, the stationary power supply 112, the customer
side electric power system 113 and the EMS 115 are connected via the electric power
network 116 and the communication network 117. The EMS 115 performs control to stabilize
the entire system 110 by utilizing the electric power network 116 and the communication
network 117.
[0163] The electric power plant 111 generates a large amount of electric power from fuel
sources such as thermal power or nuclear power. Electric power is supplied from the
electric power plant 111 through the electric power network 116 and the like. In addition,
the battery pack 40A is installed in the stationary power supply 112. The battery
pack 40A can store electric power and the like supplied from the electric power plant
111. In addition, the stationary power supply 112 can supply the electric power stored
in the battery pack 40A through the electric power network 116 and the like. The system
110 is provided with an electric power converter 118. The electric power converter
118 includes a converter, an inverter, a transformer and the like. Thus, the electric
power converter 118 can perform conversion between direct current (DC) and alternate
current (AC), conversion between alternate currents of frequencies different from
each other, voltage transformation (step-up and step-down) and the like. Therefore,
the electric power converter 118 can convert electric power from the electric power
plant 111 into electric power that can be stored in the battery pack 40A.
[0164] The customer side electric power system 113 includes an electric power system for
factories, an electric power system for buildings, an electric power system for home
use and the like. The customer side electric power system 113 includes a customer
side EMS 121, an electric power converter 122, and the stationary power supply 123.
The battery pack 40B is installed in the stationary power supply 123. The customer
side EMS 121 performs control to stabilize the customer side electric power system
113.
[0165] Electric power from the electric power plant 111 and electric power from the battery
pack 40A are supplied to the customer side electric power system 113 through the electric
power network 116. The battery pack 40B can store electric power supplied to the customer
side electric power system 113. Similarly to the electric power converter 118, the
electric power converter 122 includes a converter, an inverter, a transformer and
the like. Thus, the electric power converter 122 can perform conversion between direct
current and alternate current, conversion between alternate currents of frequencies
different from each other, voltage transformation (step-up and step-down) and the
like. Therefore, the electric power converter 122 can convert electric power supplied
to the customer side electric power system 113 into electric power that can be stored
in the battery pack 40B.
[0166] Note that the electric power stored in the battery pack 40B can be used, for example,
for charging a vehicle such as an electric automobile. Also, the system 110 may be
provided with a natural energy source. In such a case, the natural energy source generates
electric power by natural energy such as wind power and solar light. In addition to
the electric power plant 111, electric power is also supplied from the natural energy
source through the electric power network 116.
[0167] Examples are explained below, but the approaches are not limited to examples described
below.
(Example 1)
[0168] A secondary battery was produced by the following procedure.
<Production of Positive Electrode>
[0169] A lithium manganese oxide (LiMn
2O
4) having a spinel structure and an average particle size of 10 µm as a positive electrode
active material, a graphite powder as an electro-conductive agent, and polyacrylimide
(PAI) as a binder were used. The positive electrode active material, the electro-conductive
agent, and the binder were mixed in a proportion of 80% by weight, 10% by weight,
and 10% by weight, respectively, and the mixture was dispersed in an N-methyl-2-pyrrolidone
(NMP) solvent to prepare a slurry. The prepared slurry was coated onto both of reverse
surfaces of a Ti foil having a thickness of 12 µm as the positive electrode current
collector, and the coat of applied slurry were dried to form positive electrode active
material layers. The positive electrode current collector onto which the positive
electrode active material layers were formed was subjected to pressing to produce
a positive electrode having an electrode density of 3.0 g/cm
3 (excluding the current collector).
<Production of Negative Electrode>
[0170] An Li
4Ti
5O
12 powder having an average secondary particle size (a diameter) of 15 µm as a negative
electrode active material, a graphite powder as an electro-conductive agent, and PAI
as a binder were used. The negative electrode active material, the electro-conductive
agent, and the binder were mixed in proportions of 80% by weight, 10% by weight, and
10% by weight, respectively, and the mixture was dispersed in an N-methyl-2-pyrrolidone
(NMP) solvent to prepare a slurry. The obtained slurry was coated onto a Zn foil having
a thickness 50 µm as the negative electrode current collector, and the coat of applied
slurry was dried to form a negative electrode active material layer. When the slurry
was coated onto the Zn foil, the slurry was coated onto only one surface of the Zn
foil for the portion which had become located on the outermost periphery of the electrode
group in the produced negative electrode, and the slurry was coated onto both of reverse
surfaces of the Zn foil for the other portions. The negative electrode current collector
onto which the negative electrode active material layer was formed was subjected to
pressing to produce a negative electrode having an electrode density of 2.0 g/cm
3 (excluding the current collector).
<Production of Electrode Group>
[0171] The positive electrode produced as above, a non-woven separator formed of a cellulose
fiber having a thickness of 20 µm, the negative electrode produced as above, and another
non-woven separator were stacked in this order to obtain a stack. Next, the stack
was spirally wound such that the negative electrode had become located at the outermost
periphery to produce an electrode group. The electrode group was heat-pressed at 90°C
to produce a flat electrode group. The obtained electrode group was housed in a container,
which was a thin metal can formed of stainless steel having a wall thickness of 0.25
mm. As the metal can, used was a can provided with a valve capable of releasing gas
when the inner pressure reaches 2 atmospheric pressure or more.
<Preparation of electrolyte>
[0172] A solution was prepared by dissolving 12 mol/L of LiCl in water. NMP was mixed into
this solution so as to be a ratio of 10% by volume to the whole solution. To the obtained
electrolyte, 1% by volume of polyoxyalkylene alkyl ether was added.
[0173] Next, an evaluation method for the produced cell will be described. The results are
shown in Table 1.
<Storage performance evaluations
(Initial discharge capacity)
[0174] The secondary battery was charged to 2.6 V at a constant current of a 0.5C rate (conversion
based on the negative electrode active material) in an environment at 25°C after the
lapse of the standby time of 3 hours from the assembly of the cell. Then the secondary
battery was discharged to 2.2 V at 1C to measure initial discharge capacity.
(Self-discharge rate)
[0175] After the measurement of the initial discharge capacity, charge and discharge of
a predetermined number of cycles was repeated at a 1C rate. The secondary battery
was then charged to 2.6 V at a constant current of a 1C rate (conversion based on
the negative electrode active material). The charged battery was left standing for
24 hours and then discharged to 2.2 V at 1C. As an index of the storage performance,
a self-discharge rate (%) was calculated from the following formula. The self-discharge
rate calculated by the following formula shows that the lower the value, the better
the storage performance.
<Calculation of presence proportion of compound containing element A>
[0176] The proportion of presence of the compound containing element A on the negative electrode
surface was calculated by using the image editing software Photo Impact and the image
analysis software Image-Pro Plus, as described in the first approach.
[0177] In each of Examples 2 to 48 and Comparative Examples 1 to 3, a secondary battery
was prepared in the same manner as in Example 1 except that changes were made in accordance
with Tables 1 to 3, namely, in the species of the negative electrode active material,
the species of the positive electrode active material, the identity of the separator,
the species of the first electrolyte, the concentration of the first electrolyte,
the species of the electrode current collector, the species and added amount of the
electrode additive, the standby time from the assembly of the cell, the initial charging-discharging
rate, and the number of charge-discharge cycles performed. Each of the secondary batteries
was evaluated, the results of which are shown in Tables 4 to 6.
[0179] Looking at Examples 1 to 48 and Comparative Examples 1 to 3, the self-discharge rate
of Examples 1 to 48 has been held lower. This is considered to be because, by the
presence of the compound containing the element A on the negative electrode surface,
the hydrogen generation in the negative electrode had been suppressed, whereby detachment
of the electrode active material layer had been suppressed, so that the charge-discharge
efficiency and the storage performance was maintained.
[0180] According to at least one approach and example described above, a secondary battery
is provided. The secondary battery includes an aqueous electrolyte, a positive electrode,
and a negative electrode, wherein a compound containing an element A is present on
at least a part of the surface of the negative electrode, the element A being at least
one selected from the group consisting of Hg, Pb, Zn, and Bi, and a proportion of
the compound containing the element A present on the negative electrode surface is
50% or more. Therefore, a secondary battery excellent in charge-discharge efficiency
and storage performance can be provided.
[0181] The present disclosure also encompasses the following approaches of batteries:
- 1. A secondary battery, comprising:
an aqueous electrolyte;
a positive electrode; and
a negative electrode, a compound comprising an element A being present on at least
a part of a surface of the negative electrode, the element A being at least one selected
from the group consisting of Hg, Pb, Zn, and Bi, and, according to scanning electron
microscopy, a region where the compound comprising the element A is present accounting
for 50% or more of the surface of the negative electrode.
- 2. The secondary battery according to clause 1, wherein the region where the compound
comprising the element A is present accounts for 70% or more.
- 3. The secondary battery according to clause 1 or 2, wherein the negative electrode
comprises a negative electrode active material.
- 4. The secondary battery according to clause 3, wherein the negative electrode further
comprises a current collector and a negative electrode active material layer disposed
on the current collector, the negative electrode active material layer comprises the
negative electrode active material, and the current collector comprises the element
A.
- 5. The secondary battery according to clause 3 or 4, wherein the negative electrode
active material comprises at least one selected from the group consisting of an oxide
of titanium, lithium titanium oxide, and lithium titanium composite oxide.
- 6. The secondary battery according to any one of clauses 1 to 5, wherein the aqueous
electrolyte comprises a surfactant.
- 7. The secondary battery according to any one of clauses 1 to 6, wherein the aqueous
electrolyte comprises the element A.
- 8. A battery pack comprising the secondary battery according to any one of clauses
1 to 7.
- 9. The battery pack according to clause 8, further comprising an external power distribution
terminal and a protective circuit.
- 10. The battery pack according to clause 8 or 9, further comprising plural of the
secondary battery,
wherein the secondary batteries are electrically connected in series, in parallel,
or in combination of in-series connection and in-parallel connection.
- 11. A vehicle comprising the battery pack according to any one of clauses 8 to 10.
- 12. The vehicle according to clause 11, wherein the battery pack is configured to
recover a regenerative energy of power of the vehicle.
- 13. The vehicle according to clause 11 or 12, which comprises a mechanism configured
to convert kinetic energy of the vehicle into regenerative energy.
- 14. A stationary power supply comprising the battery pack according to any one of
clauses 8 to 10.
[0182] While certain approaches have been described, these approaches have been presented
by way of example only, and are not intended to limit the scope of the claims. Indeed,
the apparatuses described herein may be embodied in a variety of other forms; furthermore,
various omissions, substitutions and changes in the form of the apparatuses described
herein may be made.